1. Field of the Invention
This invention relates to a photographic lens of a great aperture ratio, and in particular to a short distance correcting system therefor.
2. Description of the Prior Art
Photographic lenses of a great aperture ratio suffer from great deterioration of the aberrations for short distance objects and therefore cannot be said to be sufficient in short distance photographing performance, and have had a difficulty even for the order of 1/10 times in respect of the photographing performance during the opening of the stop. Therefore, to prevent deterioration of the short distance photographing performance, various short distance correcting systems have been adopted and attempts to improve the short distance photographing performance have been made. However, the balance of astigmatism and coma has been still insufficient and further, there has been the disadvantage that the fluctuation of the lateral chromatic aberration from the infinity photography to the closest distance photography is great due to the fact that a short distance correcting system has been practised.
For example, in the short distance correcting system disclosed in Japanese Laid-open Patent Application No. 138823/1975, the rearmost positive lens component of the lens system is moved at a speed lower than the movement speed of the entire system, and in the lens of a great aperture ratio disclosed in Japanese Laid-open Patent Application No. 75613/1981, the rearmost positive lens component of the lens system is fixed and only the lens units forward thereof are made movable and, in both of these systems, the air space immediately forward of the rearmost lens component of the lens system is made great at the short distance. In these systems, however, it is easy to reduce the amount of coma of the group of light rays on the lower side (outer side) of the principal ray, but it is difficult to well balance the coma resulting from the group of light rays on the upper side (inner side) of the principal ray and also great fluctuation of lateral chromatic aberration has been unavoidable. Also, a correcting system whereby the stop space is enlarged in a lens system comprising a first and a second positive lens unit with a stop disposed therebetween as is disclosed in Japanese Laid-open Patent Application No. 107209/1981 is most effective for correction of a fluctuation of lateral chromatic aberration, whereas in such system, conversely to the above-described systems, correction of the coma resulting from the group of light rays on the upper side (inner side) of the principal ray has been possible but correction of the coma resulting from the group of light rays on the lower side (outer side) of the principal ray has been difficult.